The present invention relates to immunotherapeutic agents. In particular, it relates to agents that can be used to prevent or treat a condition characterised by the presence of unwanted cells, such as tumours or other disease causing cells.
Immunotherapeutic strategies for targeting malignant disease are an active area of translational clinical research, and have been for several decades. The current models dictate that cancer represents either a functional or constitutional immunodeficiency which can be treated with immunotherapeutic manipulation of the host. These efforts can be broadly classified into 2 groups. The first serves to augment or support endogenous anti-tumour immunity through measures such as vaccination, cytokine support (IL-2, IFNγ) or reducing immunosuppressant environment (ipilimumab) whilst the second seeks to restore an absolute deficiency with components of a functional immune response (passive immunotherapy with antibodies, TCR transfer, Stem Cell Transplantation and adoptive immunotherapy). These approaches are unified by the argument that a highly effective functional anti-tumour immune response is indeed possible. Although irrefutable evidence exists for an effective anti-tumour immune response in some cases, this central pillar of tumour immunology is overwhelmingly countered by the current clinical reality that despite great efforts, no effective immunotherapeutics are available for the majority of patients with cancer. Almost all cancer vaccination trials have provided negative results, with those providing positive data most frequently demonstrating a small effect. The reality is that therapeutic antibodies, with a few exceptions, offer very modest clinical benefit in the area of oncology.
If a therapeutic strategy could be developed which can efficiently molecularly re-direct an endogenous cytotoxic anti-viral immune response to instead target malignant tissue, this may afford a new powerful and safe approach to treat malignant disease.
The majority of cytotoxic therapeutic antibodies rely on immunological effector mechanisms to deliver their anti-cancer effect such as complement dependent cytotoxicity (CDC) and Antibody Dependent Cellular Cytotoxicity (ADCC). Importantly, all cells (both healthy and malignant) have numerous mechanisms to limit attack by the immune response to avert autoimmunity. This is evident in the context of autoimmune disease where high levels of tissue-reactive antibodies, which although frequently evoke organ inflammation, rarely induce complete organ destruction. Indeed, autoimmune diseases where complete tissue destruction is observed, such as diabetes mellitus, are known to be dependent on CTL responses rather than antibody-directed mechanisms.
To improve upon the poor efficacy of therapeutic antibodies, immunoconjugates (radionuclides/toxins) and engineered antibodies which better engage with the cytotoxic effector mechanisms (e.g. glycoengineering) have been used. However clinical trials of such agents remain largely disappointing and are plagued by toxicity. One example is antibody-drug conjugates (ADCs) that have been developed to selectively target anti-tumour agents to tumours (see U.S. Pat. No. 5,773,001; U.S. Pat. No. 5,767,285; U.S. Pat. No. 5,739,116; U.S. Pat. No. 5,693,762; U.S. Pat. No. 5,585,089; US 2006/0088522; US 2011/0008840; U.S. Pat. No. 7,659,241; Hughes (2010) Nat Drug Discov 9: 665, Lash (2010); In vivo: The Business & Medicine Report 32-38; Mahato et al (2011) Adv Drug Deliv Rev 63: 659; Jeffrey et al (2006) BMCL 16: 358; Drugs R D 11(1): 85-95). ADCs generally comprise a monoclonal antibody against a target present on a tumour cell, a cytotoxic drug, and a linker that attaches the antibody to the drug. However, only a few ADCs are currently in the late stage of clinical development, and of those that are, clinical success has proven elusive.
WO 95/17212 describes conjugates consisting of peptidic T cell antigens and cell binding partners and their use in re-directed immunotherapy. The conjugates comprise a binding partner with selectivity for target cells and a T cell antigen, and are said to induce specific cytotoxicity of T cells in the treatment of cancer, autoimmune diseases, diabetes or allergic diseases. The conjugates are said to be internalised into target cells following binding of the binding partner to surface receptors, and the T cell antigen is processed from the conjugate and expressed on the cell surface in the form of a complex with MHC molecules. Cytotoxicity of T cells for the target cells is thereby induced.
However, which binding partners enable internalisation and hence subsequent presentation of the T cell antigen, and which do not, is difficult to predict from WO 95/17212. Further, the conjugates described in WO 95/17212 do not efficiently target the MHC Class-I antigen processing pathway. An advantage of the MHC Class-I pathway is that, unlike MHC Class-II molecules, MHC Class-I molecules are present on all cell types.
Smith et al (J Immunol 169: 99-107, 2002) describe the use of ricin to deliver cytotoxic T cell epitopes into the MHC Class I pathway of tumour cells, such that they are subsequently lysed. However, ricin is highly toxic and since it can bind to most cell types, it is not selective for tumour cells.
Thus, there remains a demand for more effective immunotherapeutic agents with greater efficacy and lower toxicity.